1. Field of the Invention
This invention relates to zoom lenses, and particularly to focusing apparatuses for lens systems having zoom members.
2. Description of the Prior Art
Most ordinary zoom lenses are focused by adjusting a focusing lens in front of the zoom variator lens so as to maintain a focus once it is established. Incorporating automatic focusing or AF devices in such zoom lenses causes problems in that the first or front lens members are generally heavy and the automatic operating mechanism must apply a great deal of power to move these members. Therefore, in automatically focused zoom lenses designers prefer to focus with a rear lens member, or the least bulky lens member possible. On the other hand, rear focused zoom lenses require changing the position of the focusing member when the focal length of the entire system is changed during zooming in order to continue focusing on one and the same object distance. This makes it necessary to undertake whatever measures are suitable for detecting the change in the focal length for each shift in the zoom position.
When adjusting the focus with lenses in the rear of a zoom lens assembly, or with lenses that are nearer to the photosensitive material, a number of new factors must be taken into consideration.
For example, automatic focusing devices may be divided into two classes. One of these uses an optical system separate from the photographic lens in routing light from the object to be photographed to a light sensor on whose output the object distance is evaluated. This is the so called outside-the-lens type. In the other, the light passing through the photographic lens is split off and directed toward the light sensor whose output signal is used to determine the object distance. This is called the through-the-lens (TTL) automatic focusing (AF) type. The latter avoids parallax between an image of an object in the picture frame and an image of the same object in the viewfinder. The latter system is generally the preferred type of focusing.
However, even the TTL-AF system raises problems as to what part of the zoom lens to use in measuring the object distance, or where to position the diaphragm.
For example, such difficulties would arise with video camera, cine cameras, or other similar photographic instruments which operate continuously when the focusing system adjusts the lens nearest to the object to be photographed, i.e., the front most member in the entire system. The electrical energy necessary to drive the bulky front focusing member places a severe drain on the largely limited energy store.
Also, the positioning of the diaphragm relative to the range finding element constitutes another important factor in determining how to control the operation of the diaphragm and in limiting the space which the range finding element occupies. When the diaphragm lies between the focusing lens member and the range finding element, the light rays reflected from the object and entering the focusing member may be mutilated depending upon the size of the aperture before reaching the range finding element.
Moreover, a TTL-AF range finding system of the above type operates with light from the object through the diaphragm aperture. It is thus susceptible to the depth of field as the latter varies with the diaphragm opening. More specifically, with a small aperture, the depth of field is relatively large. Thus, even if the position of the focusing lens deviates more or less from a sharp focusing position, the output signal from the range finding element falls within the limits representing an acceptable range of image sharpness. On the other hand, when the aperture is relatively large, the depth of field is shallow. Thus, a slight deviation from sharp focus results in a large variation in the range finder output signal beyond the band of values for sharp focus. Thus, because the light rays reaching the range finding element through the diaphragm affect the depth of focus, the accuracy of the range finder output signal may decrease.